Iron ion control in lead coating bath



United States Patent IRON ION CONTROL IN LEAD COATING BATH Fred E. Kendall, Chagrin Falls, and James R. Knsa, Maple Heights, Ohio, assignors to Steel Ceilings, Inc., Cleveland, Ohio, a corporation of Ohio No Drawing. Application June 13, 1952, Serial No. 293,420

10 Claims. (Cl. 117-113) This invention relates as indicated to iron ion control, and more particularly to' a method of controlling the concentration of iron ion in chemical baths for the coating of ferrous articles with other metals through the medium of a chemical displacement reaction.

There have been occasional suggestions in the past that a protective lead coating might be deposited on ferrous articles through a chemical displacement reaction, but no process of this type has been found to be commercially feasible until very recently. In the copending application of Fred E. Kendall and Henry A. Wagner Lead Coating Process and Material, Serial No. 293,421, filed even date herewith, there is disclosed a novel bath and process whereby a lead'coating of excellent quality may be deposited upon ferrous articles by means of a chemical displacement reaction. As explained in such co-pending application, however, there is a gradual build-up of iron ion concentration in thebath which eventually would require that such bath be discarded since when such iron ion concentration becomes high it tends seriously to interfere with the op eration of the bath and the deposition of a satisfactory lead coating on the work.

. It is accordingly a principal objectof our invention to provide a method of controlling iron ion concentration of chemical baths to prevent such iron ion from interfering with the intended operation of such baths.

It is a further and particular object ofthis invention to provide a method of'iron ion control effective to control the iron ion concentration of a chemical displacement bath containing in solution the lead salt of one or more aliphatic poly-basic hydroxy acids such as malic, tartaric, citric, maleic, and di-glycolic acid.

Other objects of this invention 'will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed. i

As disclosed and claimed in said co-pending application of Fred E. Kendall and Henry A. Wagner, the lead salts of the aliphatic poly-basic hydroxy acids may be dissolved in the desired quantities in water by the addition of ammonium or alkali metal salts of these same tion. The pH of the solution should be less than 7 and the solution will also desirably be heated to obtain best results. Instead of immersing the article in such solution or bath, the article may be sprayed or flowcoated with the solution.

While a usable lead protective coating can be obtained without the employment of special addition agents, much less care need be exercised when addition agents of the type commonly added to electroplating baths for the purpose of ensuring sound metallic deposits are employed. Typical suitable addition agents are listed in said co-pending application and include carbohydrates such as the simple sugars, animal and vegetable proteins such as casein andbone glue, and zein, and some organic acids such as tannic acid and pyrogallic acid. The concentration of any such addition agent employed is very small, usually in the range of 0.1 to 1 gram per liter of solution, and is effective to inhibit grain formation in well-known manner.

In said co-pending application, it is taught that for best results the pH of the solution should be maintained between approximately 4.5 'and approximately 6.5, and desirably between 5.4 and 6.0. Best results obtained with typical lead salts of aliphatic poly-basic hydroxy acids, notably lead citrate, were achieved when the pH was maintained in a relatively narrow range of from 5.6 to 5.8. The rate of deposition of the lead coating is considerably increased by raising the temperature of the solution. While the bath may be successfully operated at temperatures ranging from F. to boiling, a temperaturein the range of from about to somewhat below boiling will ordinarily be more feasible, and a temperature range of from 160 F. to 210 F. is preferred.

The maximum rate of deposition of a high quality lead coating is obtained when the solution is heated as indicated and the pH maintained within the preferred range. A considerable increase in rate can also be effected by increasing the lead ion concentration which will ordinarily be from about 3 to about 50 grams per liter. It was found that from a standpoint of operational economy and efliciency, a concentration of approximately 15 grams per liter of lead ion and about.

grams per liter of sodium citrate was most desirable. About 1 gram per liter of animal bone glue should be incorporated as an addition agent although considerable fluctuation either way is permissible. For further details of the operation of such lead coating bath, reference may be had to said co-pending application.

While excellent results are obtainable from the employment of the above-described solution, the quality of the lead deposit eventually was adversely affected by the accumulation of iron ion in the coating solution resulting from the chemical displacement reaction. When a concentration of about 20 grams per liter of ferrous iron or of about 3 grams per liter of ferric iron was reached, the

lead deposit thereafter obtained had relatively poor adhesion to the base metal, was somewhat powdery and had .relatively poor malleability and ductility. In order that the coating bath might be operated substantially contin uously and maintained in condition to afford the highest quality of lead coating, it was obvious that some means would have to be found for coping with this iron problem.

It was found that ammonium and alkali metal fluorides and bifluorides would selectively precipitate iron in the ferric state without disturbing the lead ion concentration.

1 Accordingly, it was decided to oxidize the ferrous iron to ness on ,such article by a chemical displacement reacthe ferric state whereupon it will combine with the fluoride in a complex manner to form a precipitate of ammonium or alkali metal ferric fluoride under the free acid conditions prevailing; In order to reform citric acid from the citrate combined with the ferric iron, the bifluorides were preferred to the fluorides, hydrogen being thus made available for combining with the citrate ion, giving the effect of increased acid concentration. By adding lead oxide as the lead ion concentration is gradually reduced, it will effectively combine with the free acid and the initial concentration of the solution may be maintained. Only sufficient lead oxide is added at any one time to adjust the pH of the solution to its original value and thereby maintain equilibrium. Moreover, when the ferrous citrate becomes oxidized to ferric citrate, sodium hydroxide is apparently formed which combines with the reformed citric acid to replenish the sodium citrate used during the oxidation stage. It was found experimentally that the solution would tolerate considerable quantities of the bifluoride without disturbing the lead ion concentration. Concentrations of the bifluoride in excess of 20 grams per liter, however, are considered undesirable as tending somewhat to lower the rate of lead deposition and causing such deposit to be relatively hard.

While air alone may be employed to oxidize the iron to the ferric state, the employment of molecular oxygen as a gas or atomic oxygen obtainable from oxidizing salts such as ammonium or alkali metal nitrates and nitrites may sometimes be desirable.

The above-explained process functions equally well in the case of the other aliphatic poly-basic hydroxy acids. Furthermore, it should be understood that the ammonium or alkali metal salt employed need not necessarily be a salt of the same acid as the lead salt but may be a salt of any other water soluble aliphatic poly-basic hydroxy acid. For example, lead citrate may be solubilized in ammonium or alkali metal tartrates, or lead tartrate may be solubilized in ammonium or alkali metal citrates, maleates, etc.

The following examples of certain highly successful formulations are given by way of illustration and not by way of limitation. They show the use of certain preferred compounds in accordance with our invention.

General formulation NasCsH-O'r Cone, Gnis/L. NH4FHF ggg Pb++ Cone, Guts/L. GConciil Gm f ms. Min. Max. (Mm) 15 75 5 to 20 1 to 1 50 135 to 0. 1 t0 1 125 245 5 to 20 O. 1 t0 1 215 340 5 to 20 i 0. 1 to 1 265 420 5 to 20 O 1 t0 1 275 440 5 to 20 Preferred solution formulation Chemical Components GmsJL. Oz /Gal g i f Lead ion 15 12. 5 Sodium Cltrate 135 112. 5 Ammonium Bifluoridcuu 10 8.3 Amber glue 1 0.8

Solution make-up using citric acid and sodium hydroxide Chemical Components Grns/L. 02.] Gal. ai g 6d 8.4 52.5 110 14.6... 91.4 17. G 2 34 .c 14. 6 10. 0 1.33 8. 3 Amber bone glue 1.0 4 Gina/G... 0.8

In the above example, it will be noted that lead oxide is used as the source of lead ion because of its economy and availability. Likewise, it is cheaper to prepare the alkali metal salt of the acid from the acid and alkali metal hydroxide than to buy the manufactured salt.

When the solution has been made up to specified concentration, control to maintain its strength and keep it in continuous operation is relatively simple. The lead ion concentration and the alkali metal or ammonium salt concentration should be maintained within the ranges affording maximum rate of deposition of a high quality lead coating. This may be accomplished by periodically analyzing the solution for lead. It will occasionally be necessary to replenish the bifluoride to control the iron concentration, such iron concentration likewise being periodically determined by analysis. As in the case of electroplating baths, the addition agent should be replenished from time to time as the occasion demands.

As explained above, the iron going into solution as a result of lead reduction is oxidized to the ferric state and combines with the bifluoride present and is automatically precipitated. The ferrous and ferric ion concentrations may be maintained at almost any point found convenient, but it is most economical to allow the ferrous ion concentration to increase to a maximum of about 8 to 10 grams per liter. At this concentration, oxidation by air is quite efficient to oxidize the ferrous to ferric iron and ordinarily little, if any, additional oxidizing agent need be added. Practically all of the ferric iron reacts with the bifluoride (or fluoride) to produce an insoluble compound which readily precipitates and settles to the bottom of the tank and away from the field of chemical reaction. Periodically the solution may be filtered or decanted and the precipitate removed.

While the quality of the deposit obtained is much the same over a wide temperature range, a temperature on -the order of F. is ordinarily preferred as affording a relatively high rate of deposit under easily controlled conditions.

The great advantage of our method of control of iron ion concentration resides in the fact that the iron may thus be removed without appreciably affecting the concentration of other di-valent metallic ions such as lead, copper, nickel, cobalt, and cadmium in the various baths. Supervision of the baths is much facilitated and a more uniform coated product obtained.

The aforesaid solubilizing agents, ammonium and alkali metal salts of aliphatic poly-basic hydroxy acids such as sodium citrate, ammonium tartrate, and potassium maleate, have a chelating or complexing action on the di-valent ions such as the lead ion which would otherwise also be precipitated by the fluoride or bifluoride addition. The pH should be maintained at about 3 or above for preferential precipitation of the ferric ion. If the solution should be allowed to become too acid the precipitate will re-dissolve. If an excess of the fluoride or bifluoride were to be employed then the lead ion would also start to precipitate out. It is essential that the iron be oxidized to the ferric state for successful separation.

The aliphatic poly-basic hydroxy acids ordinarily employed to form the lead salt will of course normally be relatively low molecular weight carboxylic acids.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

We therefore particularly point out and distinctly claim as our invention:

1. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 4.5 to about 6.5, a lead ion concentration of from about 3 to about 50 grams per liter, and an operating temperature of from about 70 F. to boiling; a solubilizing agent for said lead ion capable of forming a soluble complex. therewith, and a salt effective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

2. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a Water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 5.4 to about 60, a lead ion concentration of from about to about 30 grams per liter, and an operating temperature of from about 130 F. to about boiling; a solubilizing agent for said lead ion capable of forming a soluble complex therewith, and a salt effective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

3. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 4.5 to about 6.5, a lead ion concentration of from about 3 to about 50 grams per liter, an operating temperature of from about 70 F. to boiling, and a small amount of an organic soundnessenhancing addition agent effective to inhibit grain formation; a solubilizing agent for said lead salt selected from the class consisting of the ammonium and alkali metal salts of aliphatic poly-basic hydroxy acids, and a salt effective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

4. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 5.4 to about 6.0, a lead ion concentration of from about 3 to about 50 grams per liter, an operating temperature of from about 130 F. to about boiling, and a small amount of an organic soundness-enhancing addition agent effective to inhibit grain formation; a solubilizing agent for said lead salt selected from the class consisting of the ammonium and alkali metal salts of aliphatic poly-basic hydroxy acids in an amount suflicient completely to solubilize said lead salt, and a salt effective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

5. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a Water solution of lead citrate having a pH of from about 4.5 to about 6.5, a lead ion concentration of from about 3 to about 50 grams per liter, and an operating temperature of from about 70 F. to boiling; a solubilizing agent for the lead ion capable of forming a soluble complex therewith, and a salt effective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

6. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a Water solution of lead citrate having a pH of from about 5.6 to about 5.8, a lead ion concentration on the order of about grams per liter, an operating temperature of from about 160 F. to about 210 F., and a small amount of an organic soundness-enhancing addition agent elfective to inhibit grain formation; a solubilizing agent for said lead citrate selected from the class consisting of the ammonium and alkali metal salts of citric acid, and

tration of from about 3 to about 50 grams per liter; a solubilizing agent for said lead ion capable of forming a soluble complex therewith, and a salt eflective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

8. in a method of forming a lead coating on a ferrous article wherein such article is immersed in a bath comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of less than 7 for time sufficient to deposit an appreciable lead coating on such article by a chemical displacement reaction, the method of prolonging the effectiveness of such bath which comprises oxidizing ferrous iron which enters such bath as a result of such displacement reaction to the ferric state, and precipitating the ferric iron from solution by reaction with a salt selected from the class consis f the ammonium and alkali metal fluorides and es while solubilizing such lead ion with a solubilizing agent capable of forming a soluble complex therewith.

9. in a method of forming a lead coating on a ferrous article wherein such article is immersed in a bath comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 4.5 to about .5, a lead ion concentration of from about 3 I to about 50 rams oer liter, a solubilizin a cut for such a i g a salt effective to precipitate ferric iron from solution selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

7. In a bath for the coating of ferrous articles with lead by chemical displacement comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of less than 7, and a lead ion concenlead ion capable of forming a soluble complex therewith, and an operating temperature of from about 70 F. to boiling for a time sufficient to deposit an appreciable lead coating on such article by a chemical displacement reaction, the method of prolonging the eifectiveness of such bath which comprises oxidizing ferrous iron which enters such bath as a result of such displacement reaction to the ferric state, and precipitating the ferric iron from solution by reaction with a salt selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

10. In a method of forming a lead coating on a ferrous article wherein such article is immersed in a bath comprising a water solution of a lead salt of an aliphatic poly-basic hydroxy acid having a pH of from about 5.4 to about 6.0, a lead ion concentration of from about 10 to about 30 grams per liter, an operating temperature of from about F. to about boiling, a small amount of an organic soundness-enhancing addition agent effective to inhibit grain formation, and a solubilizing agent for such lead salt selected from the class consisting of the ammonium and alkali metal salts of aliphatic polybasic hydroxy acids, for a time sufficient to deposit an appreciable lead coating on such article by a chemical displacement reaction, the method of prolonging the effectiveness of such bath which comprises oxidizing ferrous. iron which enters such bath as a result of such displacement reaction to the ferric state, and precipitating the ferric iron from solution by reaction with a salt selected from the class consisting of the ammonium and alkali metal fluorides and bifluorides.

References Cited in the file of this patent UNITED STATES PATENTS Re. 12,301 Betts Jan. 3, 1905 1,371,445 Shoemaker Mar. 15, 1921 2,111,199 Alvord Mar. 15, 1938 2,230,602 Sullivan Feb. 4, 1941 2,410,844 Signaigo Nov. 12, 1946 2,545,566 Booe Mal. 20, 1951 2,560,979 Pessel July 17, 1951 2,685,842 Gutzeit Nov. 10, 1953 FOREIGN PATENTS 130,302 Great Britain of 1919 

9. IN A METHOD OF FORMING A LEAD COATING ON A FERROUS ARTICLE WHEREIN SUCH ARTICLE IS IMMERSED IN A BATH COMPRISING A WATER SOLUTION OF A LEAD SALT OF AN ALIPHATIC POLY-BASIC HYDROXY ACID HAVING A PH OF FROM ABOUT 4.5 TO ABOUT 6.5, A LEAD ION CONCENTRATION OF FROM ABOUT 3 TO ABOUT 50 GRAMS PER LITER, A SOLUBILIZING AGENT FOR SUCH LEAD ION CAPABLE OF FORMING A SOLUBLE COMPLEX THEREWITH, AND AN OPERATING TEMPERATURE OF FROM ABOUT 70*F. TO BOILING FOR A TIME SUFFICIENT TO DEPOSIT AN APPRECIABLE LEAD COATING ON SUCH ARTICLE BY A CHEMICAL DISPLACEMENT REACTION, THE METHOD OF PROLONGING THE EFFECTIVENESS OF SUCH BATH WHICH COMPRISES OXIDIZING FERROUS IRON WHICH ENTERS SUCH BATH AS A RESULT OF SUCH DISPLACEMENT REACTION TO THE FERRIC STATE, AND PRECIPITATING THE FERRIC IRON FROM SOLUTION BY REACTION WITH A SALT SELECTED FROM THE CLASS CONSISTING OF THE AMMONIUM AND ALKALI METAL FLUORIDES AND BIFLUORIDES. 